1. Field of the Invention
The present invention is directed to programming techniques for a storage element, and in particular, to programming non-volatile semiconductor memory devices.
2. Description of the Related Art
Non-volatile semiconductor memory is popular for a number of uses, including cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memory types.
Both EEPROM and flash memory utilize arrays of storage elements to store one or more bits of data per element. Each element generally includes a floating gate that is positioned above and insulated from a channel region and a semiconductor substrate. The floating gate is positioned between source and drain regions. A control gate may be provided over and insulated from a floating gate. The threshold voltage of each memory transistor is controlled by the amount of charge that has remained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before a transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate. Many EEPROMs and flash memories have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states. Such memory cells store one bit of data. Other EEPROMs and flash memory cells store multiple ranges of charge and, therefore, such memory cells can be programmed to store multiple states. Such memory cells store multiple bits of data.
The process of applying electrons or holes to a floating gate in order to program or erase a storage element has been performed by a number of different physical mechanisms. Some mechanisms which have generally shown to be viable alternatives include Fowler-Nordheim (F-N) tunneling through thin oxides, channel hot-electron (CHE) injection, and source side injection (SSI). Fowler-Nordheim tunneling is a field-assisted electron tunneling mechanism based on quantum mechanical tunneling of electrons through an oxide layer and onto a floating gate. Hot carrier and source side injection are based on the injection of energetic carriers by a large electric field injected over the energy barrier of the oxide between the substrate and the floating gate.
Fowler-Nordheim tunneling normally requires fields on the order of 10 MV/CM across the silicon/SiO2 energy barrier so that electrons can tunnel from the silicon across the SiO2 into the floating gate.
Hot carrier injection used large drain biases to create “hot” electrons. At such large drain biases, carriers that flow in the channel of a MOS transistor are heated by the large electric fields in the channel and their energy distribution is shifted higher. Through impact-ionization in the channel regions carriers gain enough energy to allow them to surmount the barrier between the substrate and floating gate. One disadvantage of channel hot electron injection is its high-power consumption. As a result, thin oxides have been used to achieve large injection fields at moderate voltages.
Source side injection (SSI) has been proposed as a lower power alternative to hot carrier injection. In this process, the channel between the source and drain regions is split into two areas controlled by different gates. On one side of the channel (the source side), a gate is biased at a condition for maximum hot-electron generation (e.g. close to the threshold voltage of the channel). At the other side of the channel (the drain side), the gate is biased to a potential that is equal to or higher than the drain voltage in order to establish a vertical field that is favorable to hot electron injection to the floating gate. As a result, the drain potential is extended toward the region between the gates by an inversion layer. The inversion layer is created under the floating gate and, in some cases, the source side gate. An effective transistor channel between source region and the inversion region is created by the area under the source side gate. Electrons are accelerated from the source made “hot” electrons in a peaked lateral field between the effective channel and the inversion region.
Hence, a mechanism which allows the use of low power and low current programming of a non-volatile memory device is generally desirable.